Fixed wrongful logic for ExtractOnlyBoundaryCells in vtkExtractGeometry.

The ExtractOnlyBoundaryCells is not working properly due to some logic
errors in deciding when to extract cells if both ExtractBoundaryCells
and ExtractOnlyBoundaryCells are on. Fixed it.

Change-Id: I0574c672507ca610b1e77272d12f4e8acc3efa0c